Test plates for in vitro analysis which contain a multiplicity of individual wells or reaction chambers are commonly known laboratory tools. Such devices have been employed for a broad variety of purposes and assays as are exemplified by U.S. Pat. Nos. 3,649,464; 4,304,865; 4,276,048; 4,154,795; and U.S. Pat. No. Re. 30,562. Microporous membrane filters and filtration devices containing such microporous membranes have become especially useful with many of the recently developed cell and tissue culture techniques and assays - particularly those in the fields of virology and immunology.
Typically, a 96-well filtration plate is used to conduct multiple assays simultaneously some of which last several hours before filtration is actually performed. With such filtration plates, especially those containing microporous membranes, there is a well recognized and recurrent problem in that fluids in the wells tend to pass through the membrane by capillary action and gravity flow thereby causing a loss of contents from within the reaction well before the desired stage in the experimental design. Prevention of fluid loss by capillary action and gravity flow becomes especially important when living cells or tissues are being maintained or grown within the reaction wells. Under these circumstances, favorable media conditions for the cells or tissue must be maintained for hours or even days and any loss of fluid from the wells, however small, will affect the condition of the cells and influence the results of the assay. Prevention of fluid loss through the membrane in this manner is also vitally important when the assay utilizes very small sample volumes as reactants, such test samples often being less than 100 microliters in volume. The pendant drop that invariably forms on the underside of the microporous membrane due to such capillary action and gravity flow is typically about 50 microliters in volume and it is apparent that a fluid loss of such proportions must drastically affect the assay.
Nevertheless, insofar as is presently known, no filtration apparatus has been able to prevent this loss of fluid from the reaction well, particularly under small sample volume assay conditions.